The invention is related to the non-destructive testing (NDT) of adhesively-bonded joints which consist of two plates, usually made of metal and adhesive layer between them. For example, the adhesive joints are assembled with sheets of steel or, in some cases, aluminium sheets. The typical thicknesses of these materials are in the range of 0.7-2 mm. During the manufacturing process, adhesives or sealants are typically applied between these sheets prior to the formation of complex joints by means of spot welds, rivets or clinch flanges. Naturally, during the formation process, large forces are applied to these metals, resulting in the deformation of the mating parts. This gives rise to large-scale variations in the thicknesses of the adhesive layers. In fact, in some regions the thicknesses are often found to be less than 0.1 mm, while in others exceed 1 mm. Furthermore, uncured adhesives tend to accumulate in locations where the gap between adherends is increased, thereby leaving voids in neighbouring regions which remain in the joint even after the curing process is complete. Adhesive joints are therefore highly non-uniform in nature.
Many ultrasonic method have been proposed for testing of the adhesive joints.
Difficulties of the ultrasonic testing of the adhesive joints caused by the impedance mismatch of the materials and signal overlapping. The acoustical impedance mismatch between the adhesive and the metal—especially for steel—produces prolonged, strong reverberations of the wave in the first metal sheet. If the time delays of the wave propagation in the adhesive layer and metal sheet are approximately equal (or a multiple of each other) these reverberations sufficiently mask the small echoes returning from the second adhesive/metal or the adhesive/air interface (furthest from the transducer).
To reduce the impedance mismatch between the transducer and the metal sheet it has been proposed that the contact transducer utilize a wear resistant shoe of high impedance. In this experiment, the reverberations in the metal have been shown to be significantly decreased, enabling the detection of the resonance frequencies of the adhesive layer. However, these measurements were very sensitive to the quality of the acoustical contact. As a result, this technique was only effective when using perfectly flat and smooth surfaces.
Several ultrasonic resonance spectroscopy methods have been proposed for the evaluation of layered structures. For example, bond testers measure the frequency and amplitude of the through-thickness resonances of a system comprised of a probe coupled to the specimen. The changes in these parameters can be used to detect disbonds and voids in the joints and also to assess the condition of the bond. Detection of the defect using narrowband ultrasonic spectroscopy, is based on measurements of the electrical impedance of the transducer, which is dependent on the acoustical impedance of the inspected layered structure. In fact, a dry coupled probe with a rubber delay line has been specially designed for applications within the automotive industry. To improve the lateral resolution of this method, the probe was equipped with a special collimator in effort to narrow the ultrasound beam. Because the mechanical load produced by the delay line on the metal sheet is relatively small, the frequency of the first through thickness resonance of the structure can be readily measured and reliably related to the integrity of the joint.
For all of these techniques, the resonances of typical joints are in the low-frequency range (less than 1 MHz). As such, it is not possible to increase the frequency of ultrasound in effort to achieve better lateral resolution.
It may, however, be possible to obtain a higher resolution by employing a common pulse-echo mode. It has been proposed that disbonds at the first interface can be detected by measuring the decay rate of the reverberations in the first metal sheet. The existence of a disbond slightly increases the reflection coefficient of the ultrasonic wave at the metal/adhesive interface and hence the reverberations should decay faster in the case of good contact between the metal and adhesive. Disbonds at the second adhesive/metal interface on the other hand can be detected through phase inversion of the wave that is reflected from this interface. To reduce the amplitudes of the reverberations in the first sheet and subsequently detect the phase of this echo, a signal processing algorithm based on adaptive inverse filtering has been developed. In this algorithm, a delayed and attenuated copy of the received waveform is subtracted from the original one. If the time delay is equal to the period of reverberation and the attenuation factor is equal to the reflection coefficient at the first interface, save for the first pulse, reflections from the rear metal interface are suppressed. Thus, the echo reflected from the rear adhesive interface is obtained and its phase can be determined, whereby potential disbonds may be detected. The thickness of the adhesive layer can also be estimated using the time delay between this echo and the first pulse that is reflected from the first metal/adhesive interface. Unfortunately, in the case of thin adhesive layers, these pulses are overlapped, making phase detection quite difficult. Moreover, the inverse procedure requires an exact waveform similarity for successive pulses reverberating in the metal sheet. When strong acoustic beam divergence and mode conversion at the interfaces between the layers are present, this requirement is not satisfied.